Optical fiber cables

ABSTRACT

Cables have reduced freespace, reduced tube diameters, and reduced strength member diameters. The cables are designed to pass robustness testing such as GR-20 while using smaller amounts of raw materials to produce.

PRIORITY APPLICATION

This application is a continuation of U.S. application Ser. No.14/336,251, filed on Jul. 21, 2014, which is a continuation of U.S.application Ser. No. 14/173,274, filed on Feb. 5, 2014, now U.S. Pat.No. 8,805,142, which is a continuation of U.S. application Ser. No.13/352,773, filed Jan. 18, 2012, now U.S. Pat. No. 8,687,930, and whichis a continuation of International Application No. PCT/US2010/043222,filed Jul. 26, 2010, which claims priority to U.S. ProvisionalApplication No. 61/230,452, filed Jul. 31, 2009, the content of each ofwhich is relied upon and incorporated herein by reference in itsentirety, and the benefit of priority under 35 U.S.C. §120 is herebyclaimed.

TECHNICAL FIELD

The present disclosure relates to optical cables having reduced materialcosts while maintaining desired performance characteristics.

BACKGROUND

Fiber optic cables utilize optical fibers to transmit signals such asvoice, video and/or data information. Where fiber optic cables aresubjected to forces, the optical fibers may be stressed and attenuationof the transmitted light may result. Industry standards address genericmechanical and optical performance requirements for fiber optic cablesto ensure attenuation losses are within acceptable ranges. One suchstandard is the Generic Requirements for Optical Fiber and Optical Fiberstandard GR-20. One way to ensure compliance with GR-20 and otherstandards is to increase the bulk of the cable, such as by increasingcable diameter, jacket thickness, etc. These measures, however, increasethe cost of the cable. It is therefore important for fiber optic cablesto be constructed in a robust manner so as to satisfy industry standardswhile maintaining costs within competitive ranges.

SUMMARY

According to a first embodiment, a cable comprises a cable jacket, abuffer tube defining a cable interior, a plurality of optical fibers inthe interior, and strength members embedded in the cable jacket. Theoptical fibers can be arranged, for example, as a ribbon stack. Thecable jacket can be extruded onto the exterior of the buffer tube, andboth the cable jacket and the buffer tube can be constructed wholly orpartly from polymer materials.

According to one aspect of the first embodiment, the ribbon stackfreespace can be lower than that of conventional cables. Cablesaccording to the present embodiments with reduced ribbon stack freespacecan show minimal attenuation response and lower material costs.

According to another aspect of the first embodiment, the strength memberheight on either side of the cable can be relatively close to the buffertube inside diameter to facilitate access to the cable interior. Thestrength member height can be, for example, within 1 mm of the buffertube inside diameter.

According to yet another aspect of the first embodiment, jacket size andstrength member size can be smaller than comparable conventional cablesin order to reduce material costs.

According to yet another aspect of the first embodiment, the ratio ofthe product of elastic modulus E and total cross-sectional area A (EA)for the fibers in the ribbon stack to the product of elastic modulus Eand total cross-sectional area A of the strength members is higher thanin conventional designs at various fiber counts.

It is to be understood that both the foregoing general description andthe following detailed description present embodiments of the invention,and are intended to provide an overview or framework for understandingthe nature and character of the invention as it is claimed.

BRIEF DESCRIPTION OF THE FIGURES

The present embodiments are explained in more detail below withreference to figures which show the exemplary embodiments.

FIG. 1 is a cross section of a cable according to a first embodiment.

FIG. 2 is a plot of the ratio of jacket area to strength member area forthe cable of FIG. 1.

FIG. 3 is a plot of the ration of fiber EA to strength member EA for thecable of FIG. 1.

DETAILED DESCRIPTION

FIG. 1 is a cross section of a micromodule cable 10 according to a firstembodiment and having an outer diameter 14. The optical cable 10comprises a jacket 20 having a wall thickness 24 and an outside diameter28 (also corresponding to the cable 10 diameter). The jacket 20surrounds and contacts the outer surface of a buffer tube 30 having aninner diameter 34 and an outer diameter 38. The jacket 20 can be formedfrom, for example, a polymer material such as polyethylene. The buffertube 30 defines an interior 40 of the cable 10. The cable interior 40accommodates a plurality of optical waveguides 50. In the illustratedembodiment, the optical waveguides 50 are arranged as a stack of fiberoptic ribbons with a ribbon stack diagonal dimension 54. Pairs ofstrength members 60 are arranged on opposite sides of the cable 10 crosssection. The strength members 60 are wholly or substantially embedded inthe cable jacket 20, and may be adjacent to and/or abut the buffer tube30. In the illustrated embodiment, the strength members 60 are circularin cross-section with diameter 64 and with a strength member height 68for each pair. The jacket 20, the buffer tube 30, the ribbon stack 50,and the strength members 60 can all extend longitudinally along theentire or substantially all of the length of the cable 10.

According to one aspect of the present embodiment, the ribbon stackfreespace can be lower than that of conventional cables. Referring toFIG. 1, “ribbon stack freespace” is generally defined as the differencebetween the buffer tube 30 inside diameter 34 and the ribbon stack majordimension—in this case diagonal 54. In conventional cables, the ribbonstack freespace has historically been above 2.0 mm, with some designshaving a freespace as high as 3.38 mm According to one aspect of thepresent embodiments, the ribbon stack freespace can be less than 1.5 mm,and more particularly less than 1.0 mm. In one embodiment, the cable 10is a 48 fiber, 4.1 mm tube inner diameter 34 cable with a ribbon stackfreespace of 0.71 mm Cables according to the present embodiment withreduced freespace can show minimal attenuation response, especially whenincorporating bend-improved fibers in the ribbon stack 50. The ribbonstack 50 is free to move radially with respect to a center line 70 ofthe cable 10, so the spacing between the buffer tube 30 in general willnot be constant with respect to any of the corners of the ribbon stack50.

According to another aspect of the present embodiment, the strengthmember height 68 can be relatively close to the buffer tube outsidediameter 38 in order facilitate access to the cable interior 40. Forexample, in one embodiment, the strength member height 68 is 3.2 mm,with each strength member 60 having a diameter of 1.60 mm, and thebuffer tube inner diameter 34 is 4.1 mm. The difference between strengthmember height 68 and buffer tube inner diameter 34 can be relativelysmall—in the range of 1.3 mm or less, or more particularly in the rangeof 1.0 mm or less. Using the strength members 60 as a blade guide, thecable jacket 10 and buffer tube 30 may be shaved away from the cable 10without damaging the ribbons in the stack 50. Six strength members 60 of1.25 mm diameter, for example, with three members on each side of thebuffer tube 30, would further decrease the difference between the buffertube outer diameter 38 and the strength member height 68. Also, if thisfeature is desired in the field, strength members 60 can be spaced orseparated (in the vertical direction in FIG. 1) in order to increase theoverall strength member height 68. In the illustrated embodiment, thestrength members 60 are dielectric rigid/semi-rigid strength members,and can be glass-reinforced plastic (GRP) rods with circularcross-sections, although other materials (e.g. steel) and/orcross-sections can be used. Referring to FIG. 1, the “strength memberheight” is defined as the spacing between the outermost edges (shown asthe uppermost and lowest edges in FIG. 1) of the outermost strengthmembers on one side of the cable. In the illustrated embodiment, thestrength members 60 abut one another so the strength member height 68 isthe sum of the diameters of the strength members 60 on each side of thecable 10. It is generally preferable that the strength members 60 abutthe buffer tube 30 to prevent jacket material from coming between thestrength members 60 and the buffer tube 30.

According to another aspect of the present embodiment, by reducing thestrength member diameter 64, the jacket thickness 24 can also bereduced. For example, a 0.55 mm reduction in strength member diameter 64was achieved for the cable 10 when compared with a conventional design.This corresponds to the same jacket thickness 24 reduction in the thickportions (or, portions not overlying the strength members 60) of thejacket 20. Similar conventional cable arrangements require at least a2.80 mm jacket wall to meet minimum jacket thickness requirements.Minimum jacket thickness is the thickness of the jacket required overthe strength members 60, indicated generally by the arrow 75 in FIG. 1.The cable 10 in the illustrated embodiment has a jacket 20 of about 2.30mm thickness. The relatively thin jacket 20 significantly reducesmaterial costs for the cable 10. In an alternative embodiment, a cablehaving six 1.25 mm diameter strength members 60—three strength memberson each side—reduces the jacket thickness even further to 2.00 mm Cablesaccording to the present embodiments can be constructed to maintain asubstantially round outer diameter while retaining the required minimumjacket thickness. The jacket thickness 24 can be, for example, in therange of 2.00 mm to 2.80 mm, or more particularly in the range of 2.30mm to 2.80 mm.

Another way to characterize the relationship between the jacket 20 andthe strength members 60 is to compare the cross-sectional area of thejacket 20 with that of the strength members 60. Jacket to strengthmember area ratio data are tabulated in FIG. 2 for cables at roomtemperature. When using strength members 60 of round cross-section, thethickness of the jacket 20 is determined by the diameter of the strengthmembers 60 plus the minimum jacket thickness 75 required over thestrength members 60. In the illustrated embodiment, the strength memberdiameter 64 is 1.60 mm, with two strength members 60 on each side of thejacket 20. The minimum jacket thickness 75 is in the range of 0.7-1.0mm. Reducing the size of the strength members 60 allows a reduction injacket size, which reduces the costs of material for the cable. In thisspecification, the term “strength member area” refers to the sum of thecross-sectional areas of all of the strength members in the jacket, andthe term “jacket area” refers to the total cross-sectional area for thejacket material. Referring to FIG. 2, the conventional design (thelowest data points on the plot, indicated by diamond data points), haslower jacket area to strength member area ratios for various fibercounts. Data describing the cable 10 illustrated in FIG. 1 correspond tothe intermediate values on the plot, and are indicated by square datapoints. For a cable 10 as shown in FIG. 1, with two 1.60 mm diameterstrength members on each side of the cable, the ratio for 12-48 fibercount cables lies in the range of 6-8. For 48-72 fiber cables, the ratiolies in the range of 7-9. For 72-96 fiber cables, the ratio lies in therange of 7.5-9.5. For 96-144 fiber cables, the ratio lies in the rangeof 8-10.

Jacket area can be further reduced by using only two strength members,of 2.05 mm diameter, one on each side of the jacket 20. In FIG. 2, datadescribing this cable correspond to the highest values on the plot, andare indicated by round data points. For this embodiment, the jacket tostrength member ratio for 12-48 fiber count cables lies in the range of10-12. For 48-72 fiber cables, the ratio lies in the range of 11-13. For72-96 fiber cables, the ratio lies in the range of 12-14. For 96-144fiber cables, the ratio lies in the range of 12-15.

According to another aspect of the present embodiment, the ratio of theproduct of elastic modulus E and total area A (EA) for the fibers in theribbon stack 50 and the strength members 60 is higher than inconventional designs. In this specification, the term “fiber area”refers to the sum of the cross-sectional areas of all of the opticalfibers in the cable, including the fiber coatings, and, for ribbonizedfibers, includes the total cross-sectional area of the fibers pluscoatings in the fiber ribbons. The term “ribbon stack fiber area” couldalso be used to describe the total cross-sectional area of the opticalfibers plus coatings in the fiber ribbons. FIG. 3 is a plot of fiberarea multiplied by fiber elastic modulus (or, “fiber EA”) divided by thestrength member EA. The fiber elastic modulus E is typically calculatedto include the fiber and coating(s) applied thereto. In FIG. 3, datashowing the ratio of fiber EA to strength member EA for the cable 10illustrated in FIG. 1 is indicated by diamond data points, while datafor a conventional cable is indicated by round data points. According toone embodiment, the ratio of fiber EA (or “ribbon stack EA” forribbonized fibers) to strength member EA is at least 0.0015×fiber count.In the illustrated embodiment, the ratio is about 0.0021×fiber count.Common matrix material used to cover multiple fibers in a fiber opticribbon has a relatively low elastic modulus and is not used to calculateribbon stack fiber area or ribbon stack EA.

The interior 40 of the cable 10 can be filled with a filling compoundsuch as, for example, a waterblocking material such as thixotropic gelor grease. Gel-free designs with or without foam tapes can also be used.

It is understood in this specification that values for jacket thickness24, cable diameter 28, buffer tube inside diameter 34 and outsidediameter 38, ribbon stack diagonal 54, strength member diameter 64,strength member height 68, etc. may vary to some degree according tomanufacturing tolerances. The values in this specification may thereforebe considered to be averages for a typical cross-section of the cable.The cross-sections in the cable may not necessarily be perfect geometricshapes; for example, the illustrated circular cross-sections may havesome degree of ovality in the manufactured cable. Diameter values maytherefore be considered to the average diameter of a cross-section atany point along the length of the cable.

The cable 10 can be constructed of materials similar to Single-TubeRibbon (SST-Ribbon™) Cables available from Corning Cable Systems, Inc.of Hickory N.C. The cable 10 can include one or more ripcords (notillustrated). An armored version of the cable 10 can include metallic ordielectric armor coatings.

The present cable embodiments may utilize tensile yarns as tensionrelief elements that provide tensile strength to the cables. A preferredmaterial for the tensile yarns is aramid (e.g., KEVLAR®), but othertensile strength materials could be used. For example, high molecularweight polyethylenes such as SPECTRA® fiber and DYNEEMA® fiber, TeijinTwaron® aramids, fiberglass, etc. may also be used. The yarns may bestranded to improve cable performance.

Many modifications and other embodiments of the present invention,within the scope of the claims will be apparent to those skilled in theart. For instance, the concepts of the present invention can be usedwith any suitable fiber optic cable design and/or method of manufacture.For instance, the embodiments shown can include other suitable cablecomponents such as an armor layer, coupling elements, differentcross-sectional shapes, or the like. Thus, it is intended that thisinvention covers these modifications and embodiments as well those alsoapparent to those skilled in the art.

What is claimed is:
 1. A cable, comprising: a polymeric cable jackethaving a strength member at least partially embedded therein; aplurality of optical fibers arranged as a ribbon stack; wherein thestrength member has a strength member area and the ribbon stack has aribbon stack fiber area, wherein a product of the strength member areaand the strength member elastic modulus is a strength member EA, and aproduct of the ribbon stack fiber area and ribbon stack elastic modulusis a ribbon stack EA, and wherein the ratio of the ribbon stack EA tostrength member EA is at least 0.0015 times the number of optical fibersin the ribbon stack; wherein the plurality of optical fibers is between12-144 optical fibers, and wherein the polymeric cable jack has a jacketarea, and a ratio of the strength member area to the jacket area lies inthe range of 6-10.
 2. The cable of claim 1, wherein the polymeric cablejacket has a substantially round outer diameter and the strength memberis a set of diametrically opposed strength members of non-circularcross-sectional dimension.
 3. The cable of claim 1, wherein a thicknessof the polymeric cable jacket is less than 2.80 mm.
 4. The cable ofclaim 1, further comprising a polymeric buffer tube between the ribbonstack and the polymeric cable jacket.
 5. The cable of claim 4, whereinthe ribbon stack is located in an interior of the polymeric buffer tubesuch that the ribbon stack has a freespace of less than 1.5 mm.
 6. Thecable of claim 4, wherein the buffer tube has an inner diameter and thestrength member has a strength member height that is within 1.3 mm ofthe inner diameter of the buffer tube.
 7. The cable of claim 1, whereinthe strength member includes a first set of strength members on a firstside of the polymeric cable jacket and a second set of strength memberson an opposite side of the polymeric cable jacket.
 8. The cable of claim1, wherein the ribbon stack is free to move radially with respect to acenter line of the cable.
 9. The cable of claim 1, wherein a minimumjacket thickness over the strength member is in a range of 0.7-1.0 mm.10. The cable of claim 1, further comprising a filling compoundcomprising a waterblocking material.
 11. The cable of claim 1, furthercomprising a tension relief element for providing tensile strength tothe cable.
 12. A cable, comprising: a polymeric cable jacket having astrength member at least partially embedded therein; and a plurality ofoptical fibers arranged as a ribbon stack; wherein the polymeric cablejacket has a jacket area, the strength member has a strength memberarea, and a ratio of the strength member area to the jacket area lies inthe range of 6-10; wherein the ribbon stack has a ribbon stack fiberarea, wherein a product of the strength member area and a strengthmember elastic modulus is a strength member EA, and a product of theribbon stack fiber area and a ribbon stack elastic modulus is a ribbonstack EA, and wherein the ratio of the ribbon stack EA to strengthmember EA is at least 0.0015 times the number of optical fibers in theribbon stack; wherein a buffer tube has an inner diameter and thestrength member has strength member height that is within 1.3 mm of theinner diameter of the buffer tube.
 13. The cable of claim 12, whereinthe plurality of optical fibers is between 12-144 optical fibers. 14.The cable of claim 12, wherein the polymeric cable jacket has asubstantially round outer diameter and the strength member is a set ofdiametrically opposed strength members of non-circular cross-sectionaldimension.
 15. The cable of claim 12, wherein a thickness of thepolymeric cable jacket is less than 2.80 mm.
 16. The cable of claim 12,further comprising a polymeric buffer tube between the ribbon stack andthe polymeric cable jacket.
 17. The cable of claim 16, wherein theribbon stack is located in an interior of the polymeric buffer tube suchthat the ribbon stack has a freespace of less than 1.5 mm.